Such a technique for separating serum or plasma from blood by centrifugation using a difference in specific gravity between blood components is widely used. To accelerate the separation, various serum or plasma separating compositions have heretofore been proposed.
Compositions presently used as the serum or plasma separating compositions are those having thixotropy. Such a serum or plasma separating composition contained in a testing container can be prevented from moving during transportation or storage because it has thixotropy. Furthermore, after blood is collected in the container, a partition wall made of the serum or plasma separating composition can be formed by centrifugation. The partition wall is less likely to collapse during withdrawal of serum or plasma lying on the partition wall from the container into another container or during transportation or storage of the container.
Patent Literature 1 below discloses separating compositions in which inorganic fine powder is dispersed in a liquid resin component for the purpose of controlling the specific gravity and imparting thixotropy. Examples of the inorganic powder being used in this technique are silicon dioxide-based inorganic powders, such as silica or bentonite, and titanium dioxide-based inorganic powders. The Background Art section of Patent Literature 1 discloses, as examples of the liquid resin component, liquid resins having themselves a liquid form, such as silicone oil, α-olefin-maleic acid diester copolymers, acrylic resins and polyester copolymers.
The separating composition cannot obtain a sufficient value of initial yield stress unless the inorganic powder concentration is high. However, if inorganic powder is contained at a high concentration, the network of hydrogen bonding between inorganic powder particles is strengthened with time, whereby the value of yield stress is increased so much that the separating composition will not exhibit fluidity even when a normal centrifugal force is applied thereto. In addition, if the network density is further increased with time, the separating composition may cause a phase separation into island domains in which the inorganic powder exists at relatively high concentrations and a sea domain lean in inorganic powder. Once a phase separation occurs, the value of yield stress in the island domains are further increased and the components in the island domains become less likely to be fused again with other components in the separating composition. Therefore, during fluidization by centrifugation, the island domains may be fragmented to drift as oily droplets in the blood or the sea domain may be broken up to float as oily films on the blood. Once such oily droplets or oily films are produced, they may contaminate a measurement device to induce measurement errors.
To solve the above problems, in Patent Literatures 1 and 2 below, an organic compound, such as various surfactants including a polyoxyethylene-polyoxypropylene block copolymer and silicone-based surfactants, is contained as a thixotropy enhancer in the separating composition. Thus, the inorganic powder concentration can be reduced and the thixotropy can be stabilized over a long period of time.
However, most of the above organic thixotropy enhancers exhibit water solubility, which presents the problem of elution of the thixotropy enhancer into the blood. As a result, blood cell membranes may be damaged and cellular blood components may thereby leak out to have an adverse effect on test values. Furthermore, the absorption of water in the blood into the separating composition may be promoted to make the separating composition cloudy.